The present invention relates to a method of modifying the flow within continuous flow sedimentation centrifuges, particularly xe2x80x9cimperforate basketxe2x80x9d or xe2x80x9csolid bowlxe2x80x9d centrifuges, and thereby improving the separation performance thereof. Modification of the centrifuge or its operating conditions can produce an appropriate degree and type of mixing to cause improved separation performance.
Continuous flow sedimentation centrifuges are often assumed to operate in plug flow in modeling their separation performance. The assumption of plug flow was made in developing the widely-used sigma model [Ambler, C., Centrifuges, pp. 19-89-19-101xe2x80x9d), in Perry""s Chemical Engineers"" Handbook, Perry, R. H. and Green, D. W. eds., Sixth Edition, McGraw-Hill, New York (1984)]. Sedimentation centrifuges of widely differing geometries are expected to deviate from the ideal plug flow assumption in their behavior.
For the case of imperforate basket centrifuges, a boundary-layer flow model has been accepted in the literature as the more appropriate model. For example, Leung, in his 1998 book, xe2x80x9cIndustrial Centrifugation Technologyxe2x80x9d teaches that it has been known that a very thin boundary layer exists at the pool surface. The layer thickness is very small, typically on the order of millimeters, and is moving at a very high velocity. Injecting a small droplet of dye onto the surface of a rotating pool shows that the dye immediately shears off and spreads out into a thin film, moving at a much higher velocity as compared to that of an assumed plug flow across the annular pool. The incoming stream enters this boundary layer at one end and the effluent stream (centrate) exits the boundary layer at the other end. Below this boundary layer is a xe2x80x9cstagnant poolxe2x80x9d as observed in the reference frame of the rotating bowl. As the particles from the fast-moving boundary layer settle out into the stagnant pool, essentially the particles settle unimpeded to the bowl wall. As the particles settle from the boundary layer at the pool surface to the stagnant zone, an equivalent volume of clarified liquid is displaced. [Leung, W., Industrial Centrifugation Technology, McGraw-Hill, New York (1998)]
In imperforate basket centrifuges which operate at very high g-forces, the bulk of the liquid in the centrifuge bowl is designed to rotate as a solid body or stagnant pool while an incoming feed skims over the inner radius of the pool. [Leung, W., supra] This assumption of boundary layer flow leads to predictions of separation performance which differ markedly from those predicted by plug flow models.
While substantially boundary layer flow can be induced in an imperforate basket centrifuge by first placing a higher density solution in the bowl of the centrifuge prior to entry of a lower density tracer solution to the top surface of the pool (so the tracer solution skims across the surface of the pool), this is not what has been observed in most real world situations. Similarly, substantial plug flow can also be induced for unusual compositions. But neither boundary layer flow nor plug flow have been found to be fully accurate in predicting the real-world performance of imperforate basket centrifuges with most systems.
To develop improved imperforate basket centrifuges it has been discovered that there is a need to control the degree of mixing and thereby the flow that occurs during operation of such high g-force systems. The present invention utilizes that flow control to enhance the performance of separations performed with the equipment. For products of very high value, seemingly inconsequential product losses can add up to large amounts of revenue over time. Thus there is an immediate payback for the time and attention invested to applying the method of this invention during process development.
It must be noted that experts in the field of centrifuges generally believe that while feed solids concentration is critical to the operation of centrifuges having a cylindrical bowl geometry, e.g. imperforate basket centrifuges, the influence of temperature might be not that interesting because of the high g-forces. While there has been recognition that temperature could be an interesting parameter in low g-force systems, such would not be expected at the high g-force centrifugal accelerations found in the present invention because of high Peclet-Number conditions where the effect of diffusion was thought minimal.
Furthermore, the variations in separation performance that have been observed with the use of centrifuges were considered to be inherent in this type of separation process.
Accordingly, it is an object of this invention to provide means for controlling the extent of mixing which occurs within a sedimentation centrifuge, particularly an imperforate basket centrifuge, to improve the separation performance of the centrifuge.
Most particularly, it is an object of this invention to provide means for controlling the operation of sedimenting centrifuges, particularly those devices utilizing a generally cylindrical bowl geometry.
The foregoing and other objects, advantages and features of the invention will be apparent from the following detailed description of the invention.
The present invention is directed to a method of controlling the performance of a centrifuge having a generally cylindrical geometry. More particularly this invention is directed to a method of controlling an imperforate basket centrifuge having an imperforate rotating bowl with an inlet for feed materials and an outlet for centrate and solids, a stationary housing for rotatably mounting the bowl and having a jacket surrounding the bowl, the jacket including means for heating and cooling the bowl. The method comprises changing the temperature in the bowl to obtain a predetermined amount of mixing which will permit optimumization of the operation of the centrifuge for each particular separation.
The invention is further directed to a method of controlling the performance of an imperforate basket centrifuge (as well as similar sedimentation centrifuges with cylindrical bowl geometry) having an imperforate rotating bowl with an inlet for feed materials and an outlet for centrate and solids, a stationary housing for rotatably mounting the bowl and having a jacket surrounding the bowl, the jacket including means for heating and cooling the bowl, by adjusting the temperature in the bowl to encourage or discourage boundary layer flow as needed to enhance separation performance for a particular separation.
The present invention is further directed to a method of controlling the performance of an imperforate basket centrifuge having an imperforate rotating bowl with an inlet for feed materials and an outlet for centrate and solids, a stationary housing for rotatably mounting the bowl and having a jacket surrounding the bowl, the jacket including means for heating and cooling the bowl, by modifying the degree of mixing within the bowl by the addition of mechanical means to the bowl.
The present invention is further directed to a method of improving the performance of an imperforate basket and other similar centrifuges having an imperforate rotating bowl with an inlet for feed materials and an outlet for centrate and solids, a stationary housing for rotatably mounting the bowl and having a jacket surrounding the bowl, the jacket including means for heating and cooling the bowl, by adjusting the temperature in the jacket by an amount sufficient to increase or decrease mixing within the bowl.
The invention is further directed to a method of improving the performance of an imperforate basket centrifuge having an imperforate rotating bowl with an inlet for feed materials and an outlet for centrate and solids, a stationary housing for rotatably mounting the bowl and having a jacket surrounding the bowl, the jacket including means for heating and cooling the bowl, by adjusting the temperature in the jacket by an amount sufficient to discourage boundary layer flow.
The present invention is further directed to a method of improving the performance of an imperforate basket centrifuge having an imperforate rotating bowl with an inlet for feed materials and an outlet for centrate and solids, a stationary housing for rotatably mounting the bowl and having a jacket surrounding the bowl, the jacket including means for heating and cooling the bowl, by increasing mixing within the bowl by the addition of mechanical means to the bowl.